What you'll learn
Time is measured in seconds, minutes, hours, days, weeks, months and years, and the one rule that makes it different from every other measurement is that it does not work in tens or hundreds. There are 60 seconds in a minute, 60 minutes in an hour and 24 hours in a day, so an hour and a half is 90 minutes, not 1.5 of anything you can add straight onto a number. This topic covers telling the time in words, the 12-hour and 24-hour clocks, converting between units of time, working out how long something lasts, adding a length of time onto a starting time, reading a timetable, and using the calendar. Measurement carries 9 of the 40 items on the SEA Mathematics paper, and a time question — usually a bus, a film or a school day — appears nearly every year.
Key terms and definitions
Second, minute, hour — sixty seconds make one minute, sixty minutes make one hour, and twenty-four hours make one day.
a.m. — from midnight up to (but not including) midday. Morning.
p.m. — from midday up to midnight. Afternoon, evening and night.
12-hour clock — the way we usually speak: 3:30 p.m., quarter past seven in the morning.
24-hour clock — the way timetables are written, using four figures from 00:00 to 23:59. Half past three in the afternoon is 15:30.
Elapsed time — how long something lasted; the gap between a starting time and a finishing time.
Timetable — a table of departure and arrival times, read down the column for one journey and across the row for one stop.
Leap year — a year with 366 days instead of 365, because February has 29 days. Leap years are the years that divide exactly by 4, such as 2024 and 2028.
Core concepts
The units of time and how they join up
| This | equals this |
|---|---|
| 1 minute | 60 seconds |
| 1 hour | 60 minutes |
| 1 day | 24 hours |
| 1 week | 7 days |
| 1 year | 12 months, or 365 days |
| 1 leap year | 366 days |
| 1 decade | 10 years |
To change a large unit to a small one, multiply. To change a small unit to a large one, divide.
So 3 hours = 3 × 60 = 180 minutes. And 240 minutes = 240 ÷ 60 = 4 hours.
For mixed answers, divide and keep the remainder. 200 minutes ÷ 60 gives 3 remainder 20, so 200 minutes is 3 hours 20 minutes.
Halves and quarters of an hour
This is where most marks are lost, so learn these four by heart:
- Half an hour = 30 minutes. So 1.5 hours = 90 minutes.
- A quarter of an hour = 15 minutes. So 2.25 hours = 2 hours 15 minutes = 135 minutes.
- Three quarters of an hour = 45 minutes.
- A third of an hour = 20 minutes.
Notice that 1.5 hours is not 150 minutes and 2.25 hours is not 225 minutes. The decimal point on a clock does not behave like the decimal point in money.
Saying the time in words
Between the hour and half past, we count past. After half past, we count to the next hour.
| Written | Said |
|---|---|
| 07:15 | quarter past seven |
| 07:30 | half past seven |
| 07:45 | quarter to eight |
| 07:50 | ten to eight |
| 08:05 | five past eight |
The tricky ones are the "to" times, because the hour number goes up by one. At 07:45 you say eight, not seven, because you are counting towards eight o'clock.
The 24-hour clock
Timetables, aeroplane tickets and digital displays use four figures with no a.m. or p.m.
To change p.m. to 24-hour, add 12 to the hours: 3:30 p.m. becomes 15:30; 8:05 p.m. becomes 20:05.
To change 24-hour to p.m., subtract 12 from any hour number bigger than 12: 19:40 becomes 7:40 p.m.
Morning times keep their number but must have four figures: 6:25 a.m. is written 06:25.
Two special cases catch people out. Midday is 12:00 and is p.m., so it stays 12:00. Midnight is 00:00, and 12:15 a.m. — a quarter of an hour after midnight — is written 00:15, not 12:15.
Working out elapsed time by counting on
Never subtract times in a column the way you subtract money. Count on in steps instead, using the o'clock times as stepping stones.
To find how long it is from 10:45 to 11:30:
- 10:45 up to 11:00 is 15 minutes.
- 11:00 up to 11:30 is 30 minutes.
- Total: 15 + 30 = 45 minutes.
Anyone who writes 11:30 − 10:45 in columns gets 85, which is wrong twice over — it is not even a sensible number of minutes, since 85 minutes is more than an hour.
For a longer gap, take the whole hours first. From 09:20 to 13:05: 09:20 to 12:20 is 3 hours; 12:20 to 13:00 is 40 minutes; 13:00 to 13:05 is 5 minutes. Total: 3 hours 45 minutes.
Adding a duration to a start time
Do the hours first, then the minutes, and carry an hour whenever the minutes pass 60.
A film starts at 18:50 and lasts 1 hour 40 minutes.
- 18:50 + 1 hour = 19:50.
- 19:50 + 40 minutes: 19:50 to 20:00 is 10 minutes, leaving 30 more, so 20:30.
- It ends at 20:30, which is half past eight in the evening.
Reading a timetable
A timetable has one column for each journey and one row for each stop. Read down a column to follow one bus, and across a row to compare the times a bus reaches the same stop.
To find a journey time, take the arrival time at the last stop and count on from the departure time at the first — never subtract the column numbers.
The calendar
The months with 30 days are April, June, September and November. All the others have 31, except February, which has 28 days, or 29 in a leap year.
Days of the week repeat every 7 days. So if 1 September is a Tuesday, then 8, 15 and 22 September are all Tuesdays, because 22 − 1 = 21 and 21 divides exactly by 7. If the gap does not divide exactly, the remainder tells you how many days to count forward.
Worked examples
Example 1: How long is the journey?
Anisa's bus leaves Sangre Grande at 07:45 and reaches Port of Spain at 09:20. How long does the journey take?
Count on in steps.
- 07:45 to 08:00 is 15 minutes.
- 08:00 to 09:00 is 1 hour.
- 09:00 to 09:20 is 20 minutes.
Add up: 1 hour, plus 15 + 20 = 35 minutes.
The journey takes 1 hour 35 minutes.
Check by counting back from the answer: 1 hour after 07:45 is 08:45, and 35 minutes after that is 09:20. It agrees.
Example 2: What time does it finish?
Kern's cricket practice starts at 3:40 p.m. and lasts 2 hours 25 minutes. What time does it finish? Give the answer on the 24-hour clock.
First change the start time: 3:40 p.m. is 3 + 12 = 15:40.
Add the hours: 15:40 + 2 hours = 17:40.
Add the minutes: 17:40 to 18:00 is 20 minutes, and 25 − 20 = 5 minutes are left, so the time is 18:05.
Practice finishes at 18:05, which is five past six in the evening.
Adding 25 to 40 gives 65, and 65 minutes is 1 hour 5 minutes — that is why the hour rolled over.
Example 3: Choosing a bus from a timetable
Devon must be at school by 08:00. The bus takes 35 minutes, and after he gets off he walks for 10 minutes. The buses leave at the times in the table. Which is the latest bus he can catch?
| Bus | Leaves |
|---|---|
| A | 06:20 |
| B | 06:50 |
| C | 07:20 |
| D | 07:50 |
The whole trip takes 35 + 10 = 45 minutes.
- Bus B leaves 06:50. 06:50 + 45 minutes: 06:50 to 07:00 is 10 minutes, leaving 35, so he arrives at 07:35. That is in time.
- Bus C leaves 07:20. 07:20 + 45 minutes: 07:20 to 08:00 is 40 minutes, leaving 5, so he arrives at 08:05. That is five minutes late.
So the latest bus Devon can catch is bus B, the 06:50.
Another way to see it is to work backwards from 08:00. Take away 45 minutes: 08:00 back to 07:00 is 60 minutes, which is too far, so go back 45 minutes to 07:15. He must leave by 07:15, and the last bus before 07:15 is the 06:50. The two methods agree.
Common mistakes and how to avoid them
Subtracting times in columns. Writing 11:30 − 10:45 = 85 minutes. Fix: never do column subtraction with times. Count on through the o'clock.
Treating decimals of an hour like money. Thinking 1.5 hours is 150 minutes, or that 2.25 hours is 2 hours 25 minutes. Fix: 0.5 h = 30 min, 0.25 h = 15 min, 0.75 h = 45 min.
Getting the "to" hour wrong. Calling 07:45 "quarter to seven". Fix: after half past, you are counting towards the next hour, so say eight.
Forgetting to add 12 for p.m. Writing 3:30 p.m. as 03:30. Fix: any afternoon or evening time on the 24-hour clock starts with 12 or more.
Mishandling midnight. Writing 12:20 a.m. as 12:20 on the 24-hour clock. Fix: midnight is 00:00, so 12:20 a.m. is 00:20.
Forgetting the second part of a journey. Answering with the bus time only when there is also a walk. Fix: underline every separate length of time in the question and add them before you start.
Not converting to the unit asked for. Answering "95 minutes" when the question says "in hours and minutes". Fix: 95 ÷ 60 is 1 remainder 35, so 1 hour 35 minutes.
How parents can help at home
Time is the one measurement topic you can practise without any equipment at all, because the day supplies the questions.
Ask "how long?" out loud. When a programme starts, ask what time it will finish. When you leave the house, ask how long the drive took. Two or three of these a day is enough.
Say times both ways. If the microwave shows 19:40, ask your child to say it as twenty to eight in the evening. Digital displays mean many children rarely hear the "to" and "past" language any more, and the paper still uses it.
Work backwards from a deadline. "Church starts at half past nine and it takes 25 minutes to get there — what time must we leave?" This is exactly the shape of the hardest time item on the paper.
Use a real timetable. A bus, maxi or ferry schedule, or the television listings, gives free practice in reading down a column and across a row.
When your child is stuck, the prompt is "count on to the next o'clock first". Almost every wrong time answer comes from trying to subtract in columns, and that single instruction fixes it. If they are stuck on a duration, ask them to say the start time and the finish time out loud in words before doing anything else.
You do not need to check the arithmetic. Ask your child to say the answer back as a sentence — "the bus takes one hour and thirty-five minutes" — and ask whether that sounds sensible for the journey. If the number of minutes is 60 or more, something has gone wrong.
Exam technique for time
Decide first whether the item wants a duration (how long) or a time (when). Durations are written as hours and minutes; times are written on a clock.
Write the start and finish times down before you calculate, converting both to the 24-hour clock so there is no a.m. and p.m. confusion.
Count on through the whole hours rather than subtracting. Write the small steps down — 15 minutes, then 1 hour, then 20 minutes — because a partly correct method is easier to check.
Add every separate stage of a journey before you start: bus plus walk, cooking plus cooling, lesson plus break.
Check that no answer contains 60 or more minutes. If it does, trade 60 minutes for an hour.
Give the answer in the unit and the form the question asked for, and include a.m. or p.m. if the question used them.
Quick revision summary
- 60 seconds = 1 minute; 60 minutes = 1 hour; 24 hours = 1 day; 7 days = 1 week.
- Time is not in hundreds: 1.5 hours = 90 minutes, and 2.25 hours = 2 h 15 min.
- Large unit to small: multiply. Small to large: divide, keeping the remainder.
- After half past, count to the next hour: 07:45 is quarter to eight.
- p.m. to 24-hour clock: add 12. Midnight is 00:00; midday is 12:00.
- Find elapsed time by counting on to the next o'clock, never by column subtraction.
- 10:45 to 11:30 is 45 minutes, not 85.
- Adding a duration: hours first, then minutes, carrying an hour at every 60.
- Read a timetable down for one journey and across for one stop.
- 30 days have April, June, September and November; February has 28, or 29 in a leap year.